The present invention relates to apparatus and methods for producing high density plasma for use with plasma processing and other applications, such as high power lasers. More particularly, the invention relates to plasma production apparatus and methods that axially or radially excite whistler waves in a cylindrical plasma imbedded in a high magnetic field.
A plasma is an ionized gas. Because a plasma is a gas, it exhibits fluid characteristics that allow it to fill a desired space, assume a specific shape, or otherwise be formed for desired purposes. Because a plasma is an ionized gas, it is electrically conductive, meaning that electrical currents can flow therethrough, and the plasma can be controlled and managed to a certain extent through the application of magnetic and electric fields. Because a plasma is ionized, the ionized atoms and atomic particles therein may be chemically active or energetic, and can thereby also be used to trigger or promote a desired chemical reaction or physical process, e.g., the removal of material, as is done in plasma etching.
Most known applications for using plasma are significantly enhanced if the density of the plasma can be increased and maintained. Disadvantageously, most known techniques for making and maintaining a plasma do not result in a high density plasma. Hence, there is a need in the art for high density plasma production techniques.
There are several ways in which a plasma can be made. One of the most effective ways to make a plasma is to inject microwave energy into a gas. The energy associated with the microwave signal ionizes molecules and atoms in the gas, thereby forming the plasma Unfortunately, there is a limit to how dense the plasma can become As the plasma begins to form and become more dense, for example, it also becomes more conductive and starts to appear as an electrical short. Such an electrical short can reflect the microwave signal out of the plasma. Thus, the microwave energy may only be able to penetrate into the plasma a short distance before it is reflected out of the plasma. For this reason, the prior art teaches limiting the thickness of the plasma into which the microwave energy is injected. See, e.g., U.S. Pat. No. 4,507,588 (Asmussen et al.); U.S. Pat. No. 4, 585,668 (Asmussen et al.); U.S. Pat. No. 4,691,662 (Roppel et al.); and U.S. Pat. No. 4,727,293 (Asmussen et al.); wherein the plasma is confined to a very shallow disk.
Unfortunately, a shallow plasma disk is of limited utility for many plasma processing applications. There are at least two reasons for this. First, the "loss rate" of the plasma in a shallow disk may be higher than the loss rate for a "long" or "deep" plasma. (The "loss rate" of a plasma is the rate at which the plasma is lost either through the ions and electrons in the plasma recombining to form neutral molecules and atoms in the gas or through the ions and electrons hitting the walls of the containment vessel. In the formation of a plasma, an equilibrium point is thus reached where the ion production rate equals the ion loss rate. The loss rate may depend on such factors as the surface to volume ratio.) Second, a shallow plasma disk does not generally provide a sufficient volume of plasma for efficient use in downstream processing applications. Downstream processing applications preferably position the microwave plasma formation apart from the location where the plasma is used. See, e.g., Plasma Processing of Materials, p. 31 National Research Council, (National Academy Press, Washington, D.C., 1991). It would thus be desirable for the plasma volume positioned upstream from the location where the plasma is used to be a relatively large volume, such as a "long" or "deep" plasma cylinder, or equivalent large volume, rather than a relatively small volume, such as a shallow plasma disk. What is needed, therefore, is a technique that allows a microwave signal to be injected into a plasma volume without having the microwave signal reflected back out of the plasma due to the plasma's conductivity, thereby allowing a "deeper" or "longer" plasma volume, and thus a potentially larger plasma volume, to be formed and maintained at a location upstream from the location where the plasma is to be used.
In order to prevent the plasma from shorting out, it is known in the art to immerse the plasma in a strong magnetic field. The strong magnetic field, in general, makes it more difficult for the charged particles within the plasma to cross the magnetic field lines, and thus prevents the charged particles from shorting out. Hence, by orienting the microwave electric field used to create the plasma so that it is perpendicular to the magnetic field in which the plasma is immersed, it is possible to prevent the shorting of the plasma, and thereby improve the density limit of the plasma. U.S. Pat. No. 4,101,411 (Suzuki et al.); U.S. Pat. No. 4,401,054 (Matsuo et al.); U.S. Pat. No. 4,810,935 (Boswell); and U.S. Pat. No. 4,876,983 (Fukuda et al.) are all examples of prior art apparatus and devices that utilize microwaves and a magnetic field for various plasma processing operations.
However, even when a magnetic field is used to prevent the plasma from shorting, the injected microwave signal is still subject to damping, and such damping imposes a further density limit on the plasma. What is needed, therefore, is a technique for injecting microwaves into a plasma while increasing the density limit imposed by the damping of the microwave signal.
Two sources of damping have been identified in the prior art. The first is collisional damping, caused by collisions between electrons associated with the injected microwave energy and the ions and neutral gas molecules present in the plasma. The more dense the ions or molecules in the plasma, the more collisions that occur, and the more difficult it is for the wave to penetrate further into the plasma. Collisional damping is believed to be the factor that has heretofore limited the available plasma density in the prior art devices. See, e.g., U.S. Pat. No. 4,990,229 (Campbell et al.), where the use of an excitation frequency of 13.56 MHz for the microwave energy creates a collision frequency on the order of 2.5.times.10.sup.8 sec.sup.-1. Such a collision frequency corresponds to a plasma density of about 10.sup.19 m.sup.-3 (10.sup.13 cm.sup.-3). It would be desirable if a plasma density greater than 10.sup.13 cm.sup.-3 could be achieved.
The second source of damping is collisionless damping, also known as Landau damping. Landau damping results when the particles in the plasma have a velocity nearly equal to the phase velocity of the microwave signal injected into the plasma. The theory is that because the particles in the plasma travel with the microwave signal, they do not see a rapidly fluctuating electric field, and hence can effectively exchange energy with the microwave signal. Further, although there are electrons in the plasma that travel faster and slower than the microwave signal, the distribution of electrons is such that there are more slow electrons than fast electrons. Hence, there are more particles taking energy from the microwave signal than adding to it, and the microwave signal becomes quickly damped. Landau damping is best controlled by assuring that the phase velocity of the injected microwave signal is sufficiently larger than the thermal velocity of the particles in the plasma.
It is known in the art to use a so called "whistler wave", also known as a helicon wave, in a plasma producing apparatus. See, e.g., U.S. Pat. No. 4,990,229 (Campbell, et al.). A whistler wave propagates along the magnetic field lines. Its frequency should be much less than the electron cyclotron frequency, .omega..sub.ce. (The electron cyclotron frequency, .omega..sub.ce, is equal to eB/mc where e and m are the electron charge and mass, respectively; B is the magnetic field strength; and c is the speed of light.) In order to excite the desired whistler wave in the plasma, Campbell, et al. show particular types of antenna configurations used to surround the plasma chamber of a given plasma processing device. These antenna configurations are determined by the frequency of the rf excitation that is used, which Campbell, et al. teach, must be a low frequency, e.g. 13.56 MHz. Collisional damping thus remains the limiting factor for configurations such as those shown in Campbell et al. Hence, what is needed is a means of exciting plasma, e.g., by using whistler mode microwave signals, in a way that increases the density limit caused by collisional damping.
The present invention advantageously addresses the above and other needs.